1. Field of the Invention
These inventions relate to methods and apparatus for manipulating or transporting ions, for example multi-element ion transports, analyzers, for example quadrupole mass filters, multipole ion guides, devices for ion containment, as well as methods of making devices for controlling ions.
2. Related Art
Mass spectrometers and other analyzers have been used to determine the properties or characteristics and quantities of unknown materials, many of which are present in only minute quantities. Mass spectrometers are used in atomic and chemical analysis to determine the quantity and atomic or chemical makeup of unqualified or unknown atoms and compounds. Many such analyzers function by determining the quantity of material present in an unknown solution as a function of the mass-to-charge ratio of ions provided to the analyzer by a source of ions. The ability of the analyzer to produce reliable results depends in part on the ability of its components to get as many of the desired ions as possible from the source of ions to the detector. Additionally, the precision of the components is directly related to the types of materials used and the methods of manufacture and assembly, as well as the size of the components, in some cases. Smaller components generally require higher precision and more careful manufacture and assembly, for a given set of operating results. More precise components generally have a higher material and/or assembly cost, than other components.
One type of analyzer is a quadrupole mass spectrometer system, which generally consists of a source of ions, a quadrupole mass filter, an ion detector and associated electronics. It may also include an ion guide such as a multipole ion guide. A gaseous, liquid or solid sample ionized in the ion source and a portion of the ions created in the ion source is injected into the ion guide which transports the ions to the quadrupole mass filter. The filter rejects all ions except those in a selected mass-to-charge ratio (mass/charge) range as determined by the system electronics. (It will be understood from the context herein where the references to mass without mentioning charge refer to the mass-to-charge ratio, as appropriate, even though charge is not specifically expressed, because of the field depends on the charge of the ions). That selected mass range is usually less than 1 atomic mass unit (AMU) centered at a particular mass. Because the masses of the elements making up the sample are often unknown, the system varies the mass range from a starting mass number to an ending mass number to test for and sense particles having the masses within the mass range selected. The mass range can be as low as one AMU up to thousands of AMU. the system operates either automatically or under manual control. The mass analysis of the composition of the sample is performed by rapidly scanning the DC and RF voltage, or the frequency of the RF voltage, on the quadrupole filter, thereby scanning through the possible masses and recording the abundance of each as transmitted through the filter.
A conventional quadrupole ion guide or mass filter consists of four conductive rods arranged with their long axes parallel to a central axis and equidistant from it. The cross sections of the rods are preferably hyperbolic for a mass filter, although rods of circular cross section (“round rods”) are common. In the case of an ion guide, an RF voltage is applied to opposite pairs of poles without a DC component, so that opposite rods have the same potential and adjacent rods have equal but opposite potentials. To select which ions are rejected and which are passed through a mass filter, a selectable voltage ±(U+V cos ?) is applied on adjacent rods have equal but opposite potentials. U is the DC or offset voltage and V is the radio frequency (RF) component of the voltage applied to the quadrupole rods, at a given frequency w and time t. The field created within the region surrounded by the rods is a quadrupole field, with the electric field sensed by the ions traveling between the rods directly proportional to the distance from the central axis.
In the context of mass filter, ions injected into he entrance of the filter will exhibit oscillatory trajectories generally in the direction of the central axis (Z-axis). Those ions that oscillate too far form the central axis (in the X-axis and/or in the Y-axis directions) will, in general, not pass through the filter, while those ions that exhibit relatively short oscillatory trajectories pass from the exit of the filter and are detected. The extent of the oscillatory trajectories for a given ion mass is determined by the selected voltage. The selected voltage comes from a certain set of pre-determined voltages that are a function of the mass of the ions. the pre-determined voltage are typically developed empirically for the particular mass spectrometer configuration, and are stored in a computer or other processor memory as a look up table or equation for use during operation of the system. The magnitudes and ratio of the DC and RF components of e applied voltage can be adjusted such that only a very narrow mass range of ions will pass through the device. The narrower the mass range of the ions passing through the device, the higher the resolution, and the easier it is to distinguish ions of similar masses. Sweeping the RF voltage with a fixed RF/DC ratio will result in a mass spectrum over the range of masses selected for analysis.
Other factors affect the operation of the analyzer, such as component lengths and other dimensions, the use of vacuum, possible fringe fields at the ends of components, and the presence or absence of focusing the other elements.
Various factors also affect the cost and operation of individual components or elements. For example, the cost is typically proportional to the precision with which components are made and assembled, which in turn affects the accuracy and precision of the component. Small, precision-made components are typically more costly to make and assemble into a final component than are larger, less precise components. Mold techniques or electrode discharge machining (EDM) may be used to form very small, micro-machined components, and conventional machining, welding, brazing, and soldering can be used to form larger components. However, conventional machining and joining techniques become more difficult and expensive as the components get smaller, especially where the components are to be supported or where electrical connections are to be made. Likewise, as the number of piece parts increases, the complexity and cost of the component typically increases as well, while the precision of the components may not increase to the same extent as the complexity and the added cost has increased. Additionally, making connections with multiple wires to multiple poles or electrodes increases the cost and complexity of the component, as well as the potential discard rate.
Simple shapes for components are common and less expensive, especially for machined parts. For example, ion guides and quadrupole mass filters often use round rods as the primary elements for manipulating or transporting ions. However, hyperbolic rod cross sections may be preferred, but are more expensive and difficult to manufacture.
Additionally, the materials used in a component also affect operation, for example based on the electrical and insulting characteristics of the material. For example, stainless-still is readily used, but other metallic materials such as molybdenum, tungsten or gold coated quartz may be used as well. The materials used may depend on the available budget and the desired precision and accuracy for the component.